1. Field of the Invention
The present application relates to an image forming apparatus taking electrophotography system.
2. Discussion of the Background
Recently, information processing systems using electrophotography have been significantly developed. Among these, optical printers, which convert information into digital signals to optically record the information, have been extremely improved in terms of the quality of printing and reliability. This digital recording technology is applied to not only printers but also typical photocopiers, which leads to development of digital photocopiers. In addition, it is anticipated that a typical analogue photocopier using this digital recording technology is more and more demanded because such a photocopier has various kinds of information processing functions. Further, with the diffusion and improvement of performance of home computers, the development of a digital color printer to output color images and documents increasingly speeds up.
Higher performance and better image quality are demanded for such printers and photocopiers. Published unexamined Japanese patent application No. (hereinafter referred to as JOP). 2001-281578 describes an image forming apparatus having a multi-beam recording head to irradiate the surface of an image bearing member with multiple laser beams to deal with the demand for increase in definition and speed of image formation.
The image formation apparatus such as digital electrophotographic photocopiers and laser printers operates its image bearing member at a high linear velocity to achieve a high definition and high printing speed. Accordingly, the rotation speed of the polygon mirror in the laser beam scanning irradiation system in the image formation apparatus also rotates at a high speed and the image scanning frequency in the secondary scanning direction increases. However, the number of rotation of a polygon mirror is currently around 30,000 rpm. Further, to increase the rotation speed, there are difficult technical issues such as improvement of the bearing of the polygon mirror. Therefore, to increase the speed of image formation without increasing the rotation speed of such a polygon mirror, a method of multi-beam scanning irradiation using plural beam recording heads is adopted in which plural polygon mirrors are arranged in the secondary scanning direction to scan multiple beams per scan in the primary scanning direction.
In the multi-beam recording head system having n (n is an integer of 2 or higher) beam power sources, the number of rotation of a polygon mirror is reduced to 1/n in comparison with a system having a single beam recording head. Therefore, it is possible to increase the image formation speed n times. Also, since this provides a margin to the primary scanning speed, the scanning density can be increased. Consequently, there is a merit such that high definition images can be output at a high speed.
However, when images are formed using such a multi-beam irradiation method, a drawback occurs such that the density, breadth and size of line images and dot images may vary depending on whether adjacent beams are emitted simultaneously or separately.
FIG. 22 is a diagram illustrating the relationship between the laser lighting state and the line images formed based on the reversal development system when multiple line images are continuously written with multi-beam head scanning irradiation structured of four laser beam sources of LD1, LD2, LD3 and LD4.
When one line is formed by two beams, as illustrated in (a) of FIG. 22, the first cycle scanning is performed with LD1, LD2 and LD4 on and LD3 off. Thereafter, the second cycle scanning is performed with LD1, LD3 and LD4 on and LD2 off. LD1 and LD2 in the first scanning cycle and LD3 and LD4 in the second cycle are irradiation for forming a Line 1 and a Line 3, respectively, as illustrated in (c) of FIG. 22. In this case, the image bearing member is simultaneously irradiated with the adjacent laser beams (simultaneous irradiation).
On the other hand, the image bearing member is irradiated with LD 4 in the first cycle and LD1 in the second cycle to form a Line 2 as illustrated in (C) of FIG. 22 with a time lag therebetween, i.e., sequential irradiation, as illustrated in (b) of FIG. 22. Due to the difference between the irradiation states, the line formed in the output image by the sequential irradiation is broader than that by the simultaneous irradiation (refer to (c) of FIG. 22).
The applicants of the present application infer on the phenomenon that normally each beam has an oval form and two adjacent laser beams are partially overlapped on each other in the case of the simultaneous irradiation so that the overlapped portion on the image bearing member receives extremely strong power at one time. On the other hand, although there is no difference in the irradiations in terms of the total irradiation power, the light power on the overlapped portion on the image bearing member in the case of the sequential irradiation is relatively weak in comparison with that in the case of the simultaneous irradiation.
Image bearing members may show reciprocity failure depending on how the energy is provided thereto even when the same irradiation energy is provided. Generally, irradiation energy amount is equal to light power (value per unit time and unit area) times irradiation time. The sensitivity of an image bearing member becomes low when a light beam having a strong power is used in a short time even when the amount of the energy provided to the image bearing member is the same. Therefore, attenuation of the surface potential of the irradiated portion on the image bearing member is small.
This phenomenon is deduced as follows:    (1) A pair of charges having a positive or a negative polarity are generated in the photosensitive layer due to irradiation;    (2) Some of the charges generated during the irradiation move in the photosensitive layer to which an electric field is applied and combine with and neutralize the charge on the surface of the image bearing member to exercise the photosensitivity but part of the remaining charges extinguish when reuniting with a nearby charge having a reverse polarity;    (3) The amount of charges generated per its life time and unit space is large when the light intensity is strong even when the irradiation energy is the same. Thereby, the probability of reunion of charges becomes high. Therefore, the amount of the charges movable becomes relatively small, resulting in reduction of the sensitivity; and    (4) Also, when the intensity of the electric field applied to a photosensitive layer is low, the amount of the charges accumulated per unit space increases, which leads to rise in the probability of reunion of charges. Therefore, the amount of the charges movable becomes relatively small, resulting in reduction of the sensitivity.
As described above, the sequentially irradiated portion on the image bearing member receives relatively small irradiation power in comparison with the simultaneously irradiated portion. As a result, decrease in the sensitivity of the photosensitive layer due to the reciprocity failure is small. Therefore, the degree of attenuation of the surface potential of the image bearing member is high so that the surface potential of the irradiated portion becomes low.
The reversal development method is a developing method in which charged toner particles having the same polarity as that of an image bearing member are attached to the irradiated portion on the surface of the image bearing member. Therefore, as the potential of the irradiated portion on the surface of the image bearing member decreases, the amount of the toner for development increases. Therefore, the amount of toner attached to a sequentially irradiated portion is relatively large in comparison with that to a simultaneously irradiated portion.
An obtained toner image is transferred to a recording medium in the transfer process and thereafter fixed in the fixing process to form an image on the recording medium. In the reversal development, when toner is transferred to a recording medium, the toner scatters in the air. Thereby, the width of an obtained image is easily on the broad side. This phenomenon is referred to as toner transfer scattering. As the amount of the toner used for development increases, the area of the toner transfer scattering becomes broad, resulting in a broad line image. The transferred image is typically fixed upon application of pressure and heat in the fixing process by a fixing device such as a heating roller. During the fixing, the toner is in a flowing state and rolled. Therefore, the line image is further broadened. As the amount of the toner increases, this broadening is significant during fixing.
This is how the applicants of the present application think the line images formed on a sequentially irradiated portion become wider than those on a simultaneously irradiated portion.
The following documents describe methods of solving this drawback.
JOP 2003-205642 describes a technology in which, in addition to multiple main laser power sources, subsidiary laser power sources are provided and simultaneously and suitably emit light every time adjacent main laser power sources emit light to form images, thereby keeping the number of the light power sources simultaneously emitting light the same.
JOP 2002-113903 describes a technology in which the power of the laser emitting light is changed depending on whether adjacent laser power sources simultaneously emit light, a single laser power source emits light or power sources not adjacent to each other simultaneously emit light.
However, these technologies accompany device improvement, which leads to cost increase.
JOP 2002-107988 describes an image bearing member provided in an image forming apparatus having multiple laser beams as multi-beam image irradiation light sources to solve the drawback mentioned above. In the image bearing member, an electroconductive layer in which electroconductive particles are dispersed in the resin is provided between the electroconductive substrate and the photosensitive layer therein. However, when the electroconductive layer is in a direct contact with the photosensitive layer, the charge potential of an image bearing member tends to attenuate. Especially, when an image is formed based on reversal development, a drawback arises such that background fouling such as black spots is observed in the background portion in an image. This drawback significantly emerges while image formation is repetitively performed.
To deal with this drawback, an intermediate layer is provided to block the charges between the electroconductive layer and the photosensitive layer. However, while image formation is repetitively performed, the charges are accumulated in the intermediate layer, which leads to increase in the potential of the irradiated portion of the image bearing member. This causes a drawback such that electrostatic contrast (the difference between the voltage at the non-irradiated portion and the voltage at the irradiated portion), which is necessary to form images, becomes small.
Further, since the emitting points of the vertical cavity surface emitting laser recently developed can be arranged in a two dimensional way, the vertical cavity surface emitting laser can be used as a multi-beam light source to increase speed and density and reduce the size of a machine in comparison with a multi-beam light source using a typical end face emission laser (refer to, for example, JOP H05-294005 and P149 of No. 3 of Volume 44 of the journal of the Imaging Society of Japan, published in 2005). However, the plane emission laser has relatively a small power in comparison with a typical end face emission laser. Therefore, when the sensitivity of an image bearing member lowers while image formation is retentively performed, abnormal images and non-uniform images as mentioned above significantly occur. Therefore, various kinds of studies have been made on solving the problems involved in the multi-beam irradiation mentioned above to install a vertical cavity surface emitting laser on an image forming apparatus as a multi-beam irradiation device.
In an attempt to solve the problem of an image forming apparatus having a multi-beam image irradiation device, JOP S2005-10662 describes a technology in which an image bearing member having a photosensitive layer is provided to an image forming apparatus which forms a latent electrostatic image by scanning at least 8 laser beams emitted from a plane light emission laser array provided as an irradiation light source on the surface of the image bearing member. The specific resistance of the intermediate layer is controlled to be 108 to 1013 Ωcm when measured in the electric field of 106 V/m at 28° C. and 85% RH. However, it is found to be difficult to sufficiently deal with the drawback as mentioned above just simply by regulating the specific resistance of the intermediate layer when image formation is performed in a large amount with a linear velocity of at least 300 m/s of the image bearing member.
JOP 2005-25180 describes a technology to reduce the non-uniformity of the density by using an image bearing member in which a charge generating layer and a charge transport layer are accumulated. The sensitivity of the charge generating layer is sufficiently uniform by making the difference between maximum and the minimum of the glass transition temperature not greater than 5° C. JOP 2004-286831 describes an image bearing member of which the quantum efficiency is not less than 0.3 when the charging potential is light-decayed from 500 to 250 V as a technology to solve the drawback involved in using a plane light emission laser. JOP 2005-017381 describes an image bearing member having titanyl phthalocyanine having a light absorption of not less than 0.5 as a charge generating material.
However, in both cases, it is found to be difficult to sufficiently deal with the drawback as mentioned above when image formation is performed in a large amount with a high linear velocity of, for example, at least 300 m/s, of the image bearing member.
Further, JOP 2002-303997 describes an image bearing member having a photosensitive layer containing oxytitanium phthalocyanine for an electrophotographic image formation apparatus using a multi-beam irradiation method in which the electrophotographic process is not greater than 200 mm/s. The moving speed of the charges in the image bearing member is from 7.0×10−7 to 2.0×10−5 cm2/Vs. However, a typical image bearing member containing a known titanyl phthalocyanine has a difficulty in that such an image bearing member has a short life because residual charges easily remain in the image bearing member while the image formation process, especially the charging process and the irradiation process, is repeated. In addition, the accumulated remaining charges substantially weaken the intensity of the electric field applied to the photosensitive layer contributing to the sensitivity of the image bearing member, which promotes reciprocity failure. This causes non-uniformity between the simultaneously irradiated portion and sequentially irradiated portion mentioned above when an image is formed by a multi-beam recording in which multiple laser beams are emitted. Especially, when a plane light emission laser, which has a relatively small light power, is used as a multi-beam irradiation light source, non-uniformity in an image becomes significant due to deterioration of the sensitivity and reciprocity failure ascribable to the increase of residual charges in an image bearing member.
An image forming apparatus capable of printing at a high speed using a multi-beam is used for by far a large quantity of prints in comparison with a low or moderate speed image forming apparatus. Therefore, when the durability of an image bearing member, which is a main device in the image formation process, is low, it is inevitable that such an image bearing member is frequently replaced. This causes problems such that the substantial time to be taken to print images is long and image formation cost increases. Therefore, good durability is preferred for an image bearing member.
In addition, in an image forming apparatus taking a multi-beam irradiation system in which an image bearing member containing known titanyl phthalocyanine is provided, when the image formation is performed at a linear velocity of the image bearing member of at least 300 m/s, it is found that, when one dot or one line is plurally formed in the secondary scanning direction with adjacent multi-beams, the quality of an image pattern obtained depends on the locality therein as described above. This is considered to be because, as the irradiation time to be taken per dot decreases, the light power of a laser is strengthened, resulting in significant reciprocity failure phenomenon of the image bearing member.
Further, since the reciprocity failure phenomenon of an image bearing member is significant in irradiation under an electric field having a weak intensity, it is preferred to perform multi-beam irradiation under an electric field having a strong intensity, e.g., at least 30 V/μm to solve the drawback mentioned above involved in multi-beam irradiation. However, as described later, known titanyl phthalocyanine has various kinds of deficiencies for use under an electric field having a strong intensity. Especially, such titanyl phthalocyanine is not suitable for multi-beam irradiation under an electric field of 30 V/μm or higher. Therefore, for a high speed image forming apparatus using a multi-beam irradiation system, an image bearing member is demanded in which residual voltage does not significantly increase and the degree of reciprocity failure is light and which is free from drawbacks such as background fouling and decrease in image density even when an electric field having an intensity of 30 V/μm or higher is applied thereto.
Additionally, the functions of a high speed image forming apparatus taking digital system have been improved year by year. Therefore, let alone high durability and high stability thereof, the quality of an image is simultaneously demanded. Further, to increase the speed of color printing, a color image forming apparatus taking a tandem system having multiple image forming elements is the main stream these days. Each of the multiple image forming elements includes an image bearing member around which devices such as a charging device, an irradiation device, a developing device, a cleaning device and a discharging device for image formation are provided. In this system, respective image formation elements for yellow, magenta, cyan and black are typically installed. Each color toner image is formed at each color image formation element in parallel and overlapped on a transfer body, e.g., paper, or an intermediate transfer body to form a color image at a high speed. Therefore, such an image forming apparatus is extremely large unless the image bearing member and each device therearound are compact in size. It is inevitable that the image bearing member disposed in the center of the image formation elements has a small diameter. When an image bearing member having a small diameter has an extremely short life in comparison with an image bearing member having a large diameter, the merit in size reduction of an image forming apparatus having such an image bearing member is lost. Therefore, elongating the life of such an image bearing member in comparison with that of a typical image bearing member is recognized as a technical issue.
There are two factors which limit the elongation of the life of an image bearing member. One is electrostatic fatigue and the other is the wear of the surface layer thereof. Either of these two limiting factors is a significant issue for a currently popular organic image bearing member. The first factor is relating to the changes in the surface potential (the charging voltage and the voltage at irradiated portion) of an image bearing member while image formation process such as charging and irradiating is repetitively performed. When an image bearing member formed of an organic material is used, it is typical that the decrease in the charging voltage or the rise in the voltage at irradiated portions is a problem. The phenomenon in the second factor is that the layer disposed at the upper most surface of an image bearing member is mechanically abraded due to abrasion with a cleaning device, etc. Therefore, the thickness of this surface layer decreases, which leads to vulnerability to damage to the image bearing member, rise in the intensity of the electric field and acceleration of electrostatic fatigue. This makes the life of an image bearing member extremely short. Therefore, to elongate the life of an image bearing member, the two factors mentioned above are simultaneously eliminated.
In addition, with the realization of speed-up of the operation of an electrophotographic image forming apparatus, such an electrophotographic image forming apparatus is penetrating into the printing business field. As a result, the quality of an image and the stability level of image formation achieved by a printing machine are required for an electrophotographic image forming apparatus. As for the image quality, the definition has been greatly improved to a degree that the minimal definition of image formation is 600 dpi. With regard to the stability level of image formation, the demanded level is extremely high. This relates to the merit of electrophotography. That is, during processing such as writing and developing the same document in a massive amount, the information contained in the document can be variously changed one by one. Therefore, the stability of the system is extremely essential. It is thus natural that the image formation elements therein stably should perform image formation for repetitive use. Is it also greatly important to prevent the occurrence of an abnormal image.
The life length and the stability of an image forming apparatus are indispensable to image formation. Especially, the image bearing member included therein is the key considering its linking with other members during image formation. In every intensive attempt to develop an image bearing member, several technologies are almost successfully complete with regard to the electrostatic characteristics and abrasion of the surface thereof. For example, as for the electrostatic characteristics, charge generating materials generating optical carriers with excellent efficiency and charge transport materials having excellent mobility have been developed. When these materials are used in combination, large gain and response can be obtained in light decay. This produces effects in the entire system such as decrease of a charging potential, an amount for writing light, a developing bias and a transfer bias and elimination of a discharging process, which provides a latitude for system designing. These reduce the probability of the occurrence of hazard applied to an image bearing member so that the image bearing member itself can have an allowance.
In addition, as described above, with the advent of a high speed full color image forming apparatus, the usage of an image bearing member in an analogue or monochrome image forming apparatus has been drastically changed so that various kinds of optical writing is performed. In such usage, the occurrence of abnormal images is mostly related to an image bearing member. There are variety of causes of abnormal images, which can be largely typified into two. One is a scar on the surface of an image bearing member. The other is electrostatic fatigue of an image bearing member. The problem of abnormal images caused by a scar on the surface of an image bearing member can be mostly dealt with by improving the surface layer of an image bearing member (for example, providing a protective layer) and the device contacting the image bearing member. The problem of abnormal images stemming from electrostatic fatigue is caused by deterioration of an image bearing member. The currently most concerning issue of this type of the abnormal images is the background fouling, i.e., black spots observed in the background of an image, ascribable to reversal development, also referred to as negative positive development.
The mechanism of the occurrence of such abnormal images based on the reversal development is inferred as follows.
The reversal development is a development method of forming an image in which charged toner particles having the same polarity as that of an image bearing member are electrostatically attracted to an image portion thereof having a relatively low surface potential by irradiation on the image bearing member in comparison with the surface potential of non-image portion therearound. The charged toner is not attracted to the non-image portion (background portion), which is charged to a high potential having the same polarity as the charged toner. However, some image bearing members locally have a portion easily leaking its surface charges. That is, such an image bearing member has portions having a low voltage relative to its surround when charged. The toner is thus attached to the local portion having a low voltage, resulting in the background fouling.
There are causes to this background fouling. For example, there can be mentioned fouling and deficiency of an electroconductive substrate, dielectric breakdown of a photosensitive layer, carrier (charge) infusion from a substrate, increase in light decay of an image bearing member and generation of heat carrier in a photosensitive layer. Among these, it is possible to deal with the fouling and deficiency of an image bearing member by eliminating such substrates before forming a photosensitive layer thereon. Since this is caused by an error in a sense, this does not make an essential cause. Therefore, it is thought that this background problem can be fundamentally solved by improving the property of anti-dielectric breakdown of an image bearing member and preventing the charge infusion from a substrate and electrostatic fatigue of an image bearing member.
Technologies such that an undercoating layer or an intermediate layer is provided between an electroconductive substrate and a photosensitive layer have been proposed relating to the charge infusion from an electrostatic substrate mentioned above as one of the causes of the occurrence of the background fouling.
For example, JOP S47-6341 describes an intermediate layer containing a cellulose nitrate resin based compound, JOP S60-66258 describes an intermediate layer containing a nylon based resin, JOP S52-10138 describes an intermediate layer containing a maleic acid based resin, and JOP S58-105155 describes an intermediate layer containing a polyvinyl alcohol resin. However, such a single intermediate layer formed of a simple resin has a high electric resistance, which causes the residual potential to rise. As a result, the image density deteriorates in a negative positive development.
In addition, such an intermediate layer shows ion conductivity caused by impurities. Therefore, the electric resistance of the intermediate layer is extremely high in a low temperature and low humid circumstance. This extremely raises the residual voltage. Further, the electric resistance of the intermediate layer is lowered in a high temperature and high humid circumstance. Therefore, the background fouling tends to occur. Actually, the background fouling is not sufficiently restrained. To lower the residual voltage, it is necessary to make the thickness of an intermediate layer thin.
To deal with these problems, a technology to control the electric resistance of an intermediate layer is proposed in which electroconductive additives are added to an intermediate layer bulk. For example, JOP S51-65942 describes an intermediate layer in which carbon or chalcogen based material is dispersed in a curing resin, JOP S52-82238 describes a thermopolymeric intermediate layer in which a quaternary ammonium salt is added and an isocyanate based curing agent is used, JOP S55-113045 describes a resin intermediate layer in which a resistance controlling agent is added, and JOP S58-93062 describes an intermediate resin layer in which an organic metal compound is added. The residual voltage is reduced by simple these resin layers, but the background fouling tends to worsen. In addition, there is a problem that, when these resin layers are used in an image forming apparatus of late years using coherent light such as a laser beam, moiré is observed in images obtained.
Further, to prevent moiré and control the electric resistance of an intermediate layer at the same time, an image bearing member having a filler in its intermediate layer is proposed. For example, JOP S58-58556 describes an intermediate resin layer in which an oxide of aluminum or tin is dispersed. JOP S60-111255 describes an intermediate layer in which electroconductive particles are dispersed. JOP S59-17557 describes an intermediate layer in which a magnetite is dispersed. JOP S60-32054 describes an intermediate resin layer in which titanium oxide and tin oxide are dispersed. JOPs S64-68762, S64-68763, S64-73352, S64-73353, H01-118848 and H01-118849 describe an intermediate resin layer in which powder of borides, nitrides, fluorides and oxides of calcium, magnesium, aluminum, etc., are dispersed. In the case of such an intermediate layer in which a filler is dispersed, it is desired to increase the amount of the filler in terms of reduction of residual voltage, but it is desired to decrease the amount thereof in terms of background fouling. Consequently, it is difficult to have a good combination of reducing residual voltage and decreasing background fouling. In addition, when the content of a resin is small, the adhesive property between the intermediate layer and an electroconductive substrate deteriorates, which easily causes detachment thereof. Especially, this has a fatal effect on an image bearing member formed of an electroconductive substrate having a flexible belt form.
To deal with these problems, a technology is proposed in which an intermediate layer is formed of accumulated layers. Largely, there are two types of accumulation. One is that a resin layer 202 in which a filler is dispersed, a resin layer 203 in which a filler is not dispersed, and a photosensitive layer 204 are disposed on an electroconductive substrate 201 in this order (refer to FIG. 1). The other is that a resin layer 203 in which a filler is not dispersed, a resin layer 202 in which a filler is dispersed, and a photosensitive layer 204 are accumulated on an electroconductive substrate 201 in this order (refer to FIG. 2).
The former structure is detailed as follows. To seal off the deficiency mentioned above involved in a substrate, an electroconductive filler dispersed layer in which a filler having a low electroconductivity is dispersed is provided on an electroconductive substrate. Further, the resin layer mentioned above is provided on the electroconductive filler dispersed layer. For example, JOPs S58-95351, S59-93453, H04-170552, H06-208238, H06-222600, H08-184979, H09-43886, H09-190005, and H09-288367 describe such a structure. This structure can prevent the occurrence of moiré by the filler dispersed layer containing an electroconductive filler. In addition, it is possible to have an effect on restraining background fouling due to the resin layer provided on the filler dispersed layer. However, only the resin layer restrains the carrier infusion from the electroconductive substrate. Therefore, as in the case in which a resin layer is singly used, when the resin layer is thickened, the residual potential extremely increases. When the resin layer is thinned, the background fouling increases. Therefore, it is not satisfying in terms of achieving a good combination thereof. In addition to the insulative resin layer provided on the filler dispersion layer, the filler dispersed layer is desired to be thickened, for example, at least 10 μm, to seal off the deficiency of an electroconductive substrate. Therefore, it is difficult to restrain the occurrence of background fouling by raising the resistance of a filler contained in the filler dispersed layer because the influence of the residual potential extremely increases.
In addition, JOPs H05-100461, H05-210260 and H07-271072 describe an image bearing member in which an electroconductive layer, an intermediate layer and a photosensitive layer containing titanyl phthalocyanine crystal are accumulated. However, it is difficult to sufficiently restrain the occurrence of background fouling simply by accumulating an electroconductive layer and an intermediate layer. This is because, in addition to the cause mentioned above, the titanyl phthalocyanine contained in the photosensitive layer works as another factor to cause background fouling, which will be described later.
On the other hand, in the latter structure, a resin layer to restrain carrier infusion is provided on an electroconductive substrate and a filler dispersed layer containing a filler is provided on the resin layer. For example, JOPS H05-80572 and H06-19174 describe such a structure. In this structure, carrier infusion can be restrained by the resin layer. The filler diffusion layer accumulated thereon hardly has an effect on the residual potential even when the filler diffusion layer does not contain an electroconductive filler. Therefore, carrier infusion can be further prevented so that the latter structure is more effective than the former structure in terms of having a good combination of preventing the rise of the residual potential and reducing the background fouling.
The structure mentioned above having accumulated undercoating layers each of which has a separate function is highly effective to prevent the occurrence of moiré and background fouling and reduce the residual potential at the same time. However, since the resin layer is desired to be thickened, background fouling and residual potential tend to be greatly dependent on a combination of humidity and/or the layer thickness and a resin used in the resin layer. As a result, the structure is devoid of high stability.
Further, in addition to charge (positive hole) infusion from an electroconductive substrate to a photosensitive layer, the influence of the generation of heated carrier in the photosensitive layer is not ignorable as the cause of the occurrence of background fouling. Therefore, background fouling caused during repetitive use cannot be fully controlled without suitably selecting a charge generating material used in a charge generating layer and controlling the state of the particles thereof.
In addition, an image bearing member having a high sensitivity and a high speed responsiveness is used to deal with the issue of speed-up. It is known that an LD having a wavelength of 780 nm or an LED having a wavelength of around 760 nm is generally used as the light source and its corresponding image bearing member (charge generating material) is formed of a titanyl phthalocyanine crystal having a CuKα X ray (having a wavelength of 1.542 Å) diffraction spectrum such that at least the maximum diffraction peak is observed at a Bragg (2θ) angle of 27.3±0.2° (for example, JOP 2001-19871). This specific crystal type has an extremely high carrier generating function and therefore can be effectively used as a charge generating material contained in an image bearing member for use in a high speed image forming apparatus. However, this crystal type is unstable as a crystal and has a drawback in that the crystal form has a low stability and is vulnerable to mechanical stress and thermal stress during dispersion, etc., and easily transferred to another crystal form. The crystal form obtained after the crystal transfer has en extremely low sensitivity relative to that of the crystal form before the crystal transfer. When part of the crystal is crystalline transferred, the optical carrier generating function thereof is not fully exercised. In addition, especially abnormal images having background fouling stemming from the negative positive development easily occur while an image bearing member is repeatedly used.
Typical titanyl phthalocyanines described in JOPs 2001-19871, H08-110649, H01-299874, H03-269064, H02-8256, S64-17066, H11-5919 and H03-255456 have a strong agglomeration property. When such phthalocyanines are used in a charge generating layer, although charge infusion from an undercoating layer is restrained, reduction in charge easily occurs and dark decay tends to increase at a local portion where agglomerated or coarse particles are present. That is, background fouling becomes obvious. In addition, the purity of the titanyl phthalocyanine has a significant effect. Contaminants contained in titanly phthalocyanine cause extreme reduction in the amount of charges and increase of dark decay due to fatigue, resulting in deterioration of anti-background fouling property. Therefore, it is desired to eliminate such causes of the background fouling by controlling the dispersability and the crystal type of a titanyl phthalocyanine for use in a charge generating layer.
In addition, since images are frequently output, the quality of the output images is an important factor. To obtain an image having excellent quality, there are three issues to be dealt with, which are: (i) to form a high density latent electrostatic image formed on an image bearing member by a charging device and an irradiating device; (ii) to form a toner image true to the latent electrostatic image in the next process (development process) by a developing device; and finally (iii) to exactly transfer the toner image on the image bearing member to a transfer medium. To solve these issues, with regard to (i), there is a method of forming a latent electrostatic image by a high density writing by an irradiation device using a laser beam having a small diameter. However, when the intensity of an electric filed applied on an image bearing member is small, the optical carrier generated in a photosensitive layer spreads due to Coulomb repulsion. Therefore, the size of a dot formed does not correspond to the beam diameter. With regard to (ii), there is a method of using a toner having a small particle diameter to form a toner image true to a latent electrostatic image on an image bearing member by a developing device. When the surface potential of an image bearing member is low, the efficiency of development deteriorates. Thereby, dots formed scatters to the corresponding dots of the latent electrostatic image. With regard to (iii), there is a method of truly transferring a toner image on an image bearing member to a transfer medium by a transfer device by raising the intensity of a gap electric field to improve transfer efficiency. However, an increased intensity of the transfer electric field causes discharging to the contrary, which may cause transfer toner scattering and accelerate the fatigue of electrostatic characteristics of an image bearing member.
Among these, especially the increase in the surface potential (intensity of the electric field) of an image bearing member mentioned in (i) and (ii) causes abnormal images having background fouling when an image bearing member formed of the titanyl phthalocyanine mentioned above having a CuKα X ray (having a wavelength of 1.542 Å) diffraction spectrum such that at least the maximum diffraction peak is observed at a Bragg (2θ) angle of 27.3±0.2° is repetitively used.
FIG. 3 is a diagram illustrating how dots are formed (writing at 1,200 dpi) to the intensity of an electric field (surface potential of an image bearing member/layer thickness of a photosensitive layer) applied on an image bearing member. As illustrated in FIG. 3, to truly reproduce small dots, it is desired to have a high intensity of an electric field. In FIG. 4, the relationship between background fouling and the intensity of an electric field is illustrated. The background ranking in FIG. 4 represents the degree thereof. The larger the value of the background is, the better the degree of the background fouling is, meaning the frequency of the occurrence of background fouling is low. As seen in FIGS. 3 and 4, there is a trade off relationship between the intensity of en electric field and the background fouling ranking. To avoid background fouling, a system has been used in which the intensity of an image bearing member is typically not greater than 30 V/μm and thereby the reproduction of small dots are sacrificed in some degree. For example, JOP 2001-154379 describes that the intensity of an electric field of an image bearing member is limited in the range of from 12 to 40 V/μm to have a good combination of background fouling and reproduction of fine lines.
However, when the definition of a writing laser beam increases, it is not possible to develop written dots with good reproducibility without setting the lower limit thereof to be relatively high. In addition, with regard to background fouling, the upper limit of the intensity of an electric field varies depending on the materials (mainly charge generation material) forming an image bearing member. The titanyl phthalocyanine having a CuKα X ray (having a wavelength of 1.542 Å) diffraction spectrum such that at least the maximum diffraction peak is observed at a Bragg (2θ) angle of 27.3±0.2° has an extremely high sensitivity but has a drawback in that the titanyl phthalocyanine is not suitable on background fouling. Actually, the range of the intensity of an electric field of such a titanyl phthalocyanine is limited to around not greater than 30 V/μm.
Further, the optical carrier generating efficiency (capability) of the titanyl phthalocyanine crystal mentioned above depends on the intensity of an electric field. As the intensity of an electric field decreases, the optical carrier generating efficiency extremely worsens. Therefore, in an actual system, the advantage of the titanyl phthalocyanine crystal having the specifically high sensitivity is not fully brought out. This drawback is not greatly significant for a writing laser beam having a low definition, for example, not greater than 400 dpi, but for a high definition of late, for example, at least 600 dpi and higher, specifically, at least 1,200 dpi.
In the typical technologies, it is difficult to have a good combination of restraining background fouling and the rise in the residual voltage. To be specific, when the background fouling is restrained, it invites the rise in the residual voltage and the extreme dependency on environment. When the rise in the residual voltage is restrained, the effect on restraint of the background fouling is insufficient. As described above, background fouling is caused not only by charge infusion from an electron substrate, but also by other factors such as coarse particles contained in titanyl phthalocyanine and contaminants contained in a photosensitive layer or a charge generating layer. Furthermore, there is another factor having a great effect on the background fouling, which is the increase in the intensity of an electric field induced by the decrease in the layer thickness of an image bearing member.
Therefore, a charge transport layer or a protective layer formed as the uppermost surface layer of an image bearing member has been devised to improve anti-abrasion property. There are technologies to improve anti-abrasion property of a photosensitive layer such that (i) a curing binder resin is used in a cross linkage type charge transport layer (for example, refer to JOP S56-48637), (ii) a polymeric charge transport material is used (for example, refer to JOP S64-1728) and (iii) an inorganic filler is dispersed in a cross linkage type charge transport layer (for example, refer to JOP H04-281461). The temporary variation of the intensity of an electric field can be thus lessened by improving the anti-abrasion property of an image bearing member. Thereby, such an image bearing member has a high effect on restraint of background fouling.
However, among these, the technology mentioned in (i): the curing binder resin, is not sufficiently compatible with a charge transport material. Therefore, the residual voltage tends to rise. In addition, the residual also tends to rise due to the existence of contaminants such as non-reacted remaining group and a polymerization initiator. This leads to decrease in the image density. Further, when the polymeric charge transport material mentioned in (ii) is used, the anti-abrasion property of an image bearing member can be improved in some degree but does not reach a desired level. Further, polymerizing and refining a polymeric charge transport material is so difficult that the purity is not sufficient. Therefore, the electric characteristics between materials are not easily stable. Furthermore, there are problems relating to manufacturing such that the liquid for application has a high viscosity. In addition, in the case of the technology mentioned in (iii) where an inorganic filler is dispersed, the anti-abrasion property thereof is relatively high in comparison with that of a typical image bearing member in which a charge transport material having a low molecular weight is dispersed in an inactive polymer. However, the residual voltage rises due to charge trap, which is caused by the charge existing on the surface of the inorganic filler. This may lead to decrease in image density. Further, when the concavity and convexity of the inorganic filler and the binder resin on the surface of an image bearing member is large, the cleaning performance deteriorates, which may lead to toner filming and image flowing. These technologies (i) to (iii) have an effect on restraining background fouling but have a problem about the residual potential and cleaning performance, resulting in image deficiency. Therefore, these technologies are not fully sufficient to improve the durability of an image bearing member.
Further, an image bearing member is known which contains multifunctional acrylate monomer curing material to improve anti-abrasion property and anti-damage property (for example, refer to Japanese Patent No. (hereinafter referred to as JP) 3262488. However, in this image bearing member, there is a description in which this multi-functional acrylate curing material can be contained in a protective layer provided on a photosensitive layer of the image bearing member. This is a simple but not specific description about a charge transport material contained in the protective layer. In addition, when a charge transport material having a low molecular weight is simply contained in a cross linkage type charge transport layer, there arises a compatibility problem between the charge transport material and the curing material mentioned above. Thereby, the charge transport material having a low molecular weight precipitates and causes clouding phenomenon. Therefore, the rise in the irradiated portion voltage causes decrease in the image density and the mechanical strength weakens. Further, to manufacture this image bearing member, the monomer reacts in a state in which the polymeric binder resin is contained. Therefore, since a three-dimensional mesh structure is not fully developed and naturally the cross-linkage density is thin, this type of an image bearing member does not have a drastically improved anti-abrasion property.
As to the anti-abrasion technology relating to these, it is known that there is a charge transport layer formed of a liquid of application formed of a monomer having one or more carbon-carbon double linkages, a charge transport material having one or more carbon-carbon double linkages and a binder resin (for example, refer to JP 3194392). This binder resin is considered to have a function of improving the adhesiveness between a charge generating layer and a curing type charge transport layer and further relax the internal stress in a thick layer during curing the thick layer. The binder resin is typified into two. One has one or more carbon-carbon double linkages and is reactive to the charge transport material. The other does not have a carbon-carbon double linkage and is not reactive thereto. This type of an image bearing member is notable in that the image bearing member has a good combination of anti-abrasion property and electric characteristics. When a binder resin non-reactive to a charge transport material is used, the compatibility between the binder resin and a curing material formed in the reaction between the monomer and the charge transport material is poor so that the layer detachment tends to occur in the cross-linkage type charge transport layer, which may lead to damage or adhesion of external additives and paper dust. Further, as described above, since the three dimensional mesh structure is not fully developed, and naturally the cross-linkage density is thin, this type of an image bearing member does not have a drastically improved anti-abrasion property. Furthermore, a specific monomer for this type of an image bearing member in the description has two functional groups so that the anti-abrasion property is not sufficiently improved. In addition, when a binder resin reactive to a charge transport material is used, although the molecular weight of the curing resin increases, the number of linkages among molecules is small. Therefore, it is difficult to have a good combination of the amount and the density of the linkage of the charge transport material and the electric characteristics and anti-abrasion property are not sufficiently improved.
Additionally, it is known that there is a photosensitive layer containing a compound cured from a positive hole transfer compound having at least two chain polymeric functional groups in a molecular (for example, refer to JOP 2000-66425). This photosensitive layer can improve the density of cross linkage and thus has a high hardness. However, since the cumbersome positive hole transfer compound has at least two chain polymeric functional groups, the obtained cured compound tends to have distortion therein and a high internal stress. Thereby, the cross-linkage surface layer is vulnerable to cracking and peeling for an extended period of use. As described above, an image baring member having a cross-linkage photosensitive layer in which a charge transport structure is chemically bonded based on these typical technologies does not have sufficient comprehensive characteristics.
Since the background fouling is influenced not only by an undercoating layer but also by each layer such as a charge generating layer, a charge transport layer and a protective layer, the background fouling is not sufficiently restrained and therefore the durability of an image bearing member is not achieved without improving each layer at the same time. However, in the related typical art, there are few cases in which background fouling is restrained by each of the layers forming an image bearing member. In addition, in attempts to improve every layer at the same time, image deterioration drawbacks other than the background fouling frequently arise such that the residual potential rises, the dependency of chargeability and the residual potential on humidity increases, and filming, image blur and image deficiency tend to occur. That is, the durability of an image bearing member has not been highly improved.